A numerically controlled lathe (hereinafter referred to as the NC lathe) which includes two headstocks and tool posts disposed opposite to each other and which is capable of simultaneously machining two works attached to two main shafts with tools attached to the tool posts is known, for example, in Japanese Patent Publication No. 10-501758.
FIG. 11 is a plan view showing a schematic constitution of the NC lathe disclosed in the Japanese Patent Publication No. 10-501758.
A first headstock 520 and a second headstock 530 are disposed opposite to each other on a bed 510 of an NC lathe 5. The first headstock 520 and second headstock 530 rotatably support parallel main shafts 521, 531. These main shafts 521, 531 are arranged so that axes of the main shafts 521, 531 are eccentric. Chucks (not shown) are disposed on tip ends of the respective main shafts 521, 531, and works W1, W2 are grasped by the chucks.
The first headstock 520 is fixed to the bed 510. In the bed 510, guide rails 540 are disposed extending in a Z1-axis direction which is the same direction as that of the axis of the main shaft 531. A carriage 550 is disposed on the guide rails 540, and this carriage 550 is guided by the guide rails 540 by driving drive members such as a servo motor (not shown) to move forwards/backwards in a Z1-axis direction.
Guide rails 570 are disposed in an X1-axis direction crossing at right angles to the Z1-axis on the carriage 550. A carriage 555 is movably disposed in the X1-axis direction on the guide rails 570, and a first tool post 560 and second headstock 530 are disposed on the carriage 555. The second headstock 530 and the first tool post 560 are guided by the guide rails 540, 570 by driving the driving members such as the servo motor (not shown), and are movable in the X1-axis and Z1-axis directions.
The first tool post 560 includes a indexable/rotatable turret plane plate 561 on one side. Onto the turret plane plate 561, a plurality of tools T1 for machining work W1 held by the main shaft 521 of the first headstock 520 are attached. Moreover, by a combination of movements of the carriages 550, 555 in the Z1-axis and X1-axis directions, the tools T1 are positioned with respect to the work W1 to machine the work W1.
A second tool post 580 is disposed opposite to the main shaft 531 of the second headstock 530. An indexable/rotatable turret plane plate 581 is disposed on one side of the second tool post 580. A plurality of tools T2 for machining work W2 attached to the main shaft 531 of the second headstock 530 are attached to the turret plane plate 581. The second tool post 580 is movable along guide rails 582 disposed in parallel with the guide rails 570 in an X2-axis direction which is the same direction as that of the X1-axis.
In this NC lathe 5, the first tool post 560 and the second headstock 530 are disposed on the common carriages 550, 555. Therefore, feed of the tools T1 with respect to the work W1 corresponds to that of the tools T2 with respect to the work W2. Therefore, the same hole-making operation can simultaneously be effected with respect to two works W1, W2.
Moreover, while the tools T2 are moved in the X2-axis direction in synchronization with the movement of the tools T1 in the X1-axis direction, the movement in the X2-axis direction required for machining the work W2 by the tools T2 is added. Accordingly, it is possible to simultaneously perform different machining operations with respect to the works W1, W2.
The above-described NC lathe 5 is capable of simultaneously machining a plurality of works W1, W2 in the same or different manners, and is therefore advantageously superior in a machining efficiency, but there are the following disadvantages.
That is, since relative movement of the tools T1 and T2 with respect to the works W1, W2 in the Z1-axis and Z2-axis directions is determined by the movement of the carriage 550 in the Z1-axis direction, there is a problem that machining modes of the simultaneously machinable works W1, W2 are limited.
Moreover, even when the machining of the work W2 by either one tool, for example, the tool T2 ends in a time shorter than that of the machining of the work W1 by the tool T1, the second tool post 580 has to move following the movement of the first tool post 560 on standby until the machining of the work W1 by the tools T1 ends. This is because even when the tool T2 is changed and the work W2 is to be continuously machined with the next tool, the tool T2 cannot be detached from the work W2, and the next tool cannot be indexed. Therefore, there is a problem that there is much waste of time such as standby and the machining efficiency is bad.
Furthermore, for the NC lathe 5, the second tool post 580 can move only in an uniaxial direction of the X2-axis. Therefore, while the X1-axis is superposed on the X2-axis, machining operations such as a boring operation are carried out, and superposition error is caused. Then, there is a disadvantage that the superposition error appears as a machining quality as it is on the work. Therefore, when the superposition error is large, machining defect is easily generated, and a problem occurs that there is a possibility of a high defect ratio.
An object of the present invention is to provide an NC lathe and a method of machining the work by this numerically controlled lathe in which a variety of machining operations can simultaneously be carried out with respect to the works on the first and second main shafts and in which waste of time such as a waiting time can be reduced as much as possible and in which the machining efficiency is high. Even when the superposition error is generated during the machining operations such as the boring operation, adverse influences by the superposition error can substantially be eliminated.